Optical prism unit and manufacture thereof



June 24, 1969 .1. J. J. sTAUNToN Re. 26,617

OPTICAL PRISM UNIT AND MANUFACTURE THEREOF original Filed May 29, 1961sheet l of 5 INV EN TOR.'

ATTORNEYS.

June 24, 1969 J. J. 1. sTAuNToN Re 26,617

QPTICAL PRISM UNIT AND MNUFACTURE THEREOF Original Filed May 29. 1961Sheet of 3 IN V EN TOR.'

ATTRNEYS.

June 24, 1969 J. .1. J. sTAuNToN Re. 26,617

OPTICAL PRISM UNIT AND MANUFACTURE THEREOF Original Filed May 29. 1961Sheet -3 of 3 16k 18k 2:' l B15.' 84 F 5:16 86 Ld/82 14k T54 UnitedStates Patent O 26,617 OPTICAL PRISM UNIT AND MANUFACTURE THEREOF JohnJ. J. Staunton, Oak Park, Ill., assignor, by mcsne assignments, toColeman Instruments Corporation, Maywood, Ill., a corporation ofDelaware Original No. 3,254,556, dated June 7, 1966, Ser. No. 113,469,May 29, 1961. Application for reissue Mar. 28, 1967, Ser. No. 637,029

Int. Cl. G02b 5/04 U.S. Cl. 350-168 7 Claims Matter enclosed in heavybrackets [j appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE An optical prism unit for dispersing rays oflight. Multiple light dispersing elements are mounted in a side by sidearrangement. A beam of light may intersect the plurality of elements andbe uniformly dispersed more effectively than with a single large prism.By translating and rotating the pnit, the various` optical elements aremade to intersect the beam of light at dierent dispersive angles. Theplurality of elements may comprise elements responsive to variouswavelengths arranged in a specified order, such as in order ofincreasing wavelength responses.

Cross reference to related application This is a reissue of U.S. PatentNo. 3,254,556 granted June 7, 1966.

This invention relates to a novel form of optical prism unit whosefunction is primarily to disperse an optical beam into a spectrum, andsecondarily when so con structed, to focus this beam and thereby reducethe number of components required in the optical system. Pr marily thisunit is a form of optical prism; secondarily, it may also be a lens or acurved mirror.

The prism is commonly used in such instruments as spectrometers andspectrophotometers, where its function is to disperse an optical beamcontaining a multiplicity of wave lengths of radiation into a spectrumwherein these wave lengths are displayed in serial order for the purposeof selecting therefrom a narrow group of known wave length range formeasurement reasons. The prism as so used may be the only dispersingmeans of a single monochromator, or it may be used as an order sorter orsecondary dispersing means in a double monochromator where another prismor a grating is the primary dispersing means. As a primary dispersingmeans, the angles and surfaces of the prism must be very accuratelyworked to realize the precision of optical control necessary for highresolution, that is, accurate distinction between adjacent wave lengths.When used as a secondary dispersing means more leeway in angle andsurface precision is generally permissable, since the onus of producinghigh resolution falls on the primary dispersing component. Thesignicance of these distinctions will be more apparent later.

In the prior art, whether used as a primary or secondary dispersionmeans, the usual prism is a right prism of triangular cross section,though several other cross sections each having its peculiar advantageshave been used. The faces of the prism through which the light passesare polished accurately plane, usually to within a fringe, i.e.,microinches. The face, or faces, through which the light does not passare usually left in a line ground condition. An exception to the planeface rule is the Fery prism which has spherical curvature of thepolished faces to act as image forming surfaces for the purpose ofeliminating one or more lenses or curved mirrors which would otherwisebe needed to focus or paralleli7e the beam passing Reissued June 24,1969 through the system. A triangular prism through which the lightpasses once is known as a Bunsen prism: if the light is reflected fromthe second face of the prism or from a mirror behind the prism andreturned back through the prism, leaving by the face through which itentered, the prism is known as a Littrow prism. The details of opticalsystems using either type of prism are conventional and are not a partof this invention.

When the prior art investigators worked in the visible regions fromabout 400 to 700 millimicrons wave length, the simple prism made fromglass functioned well. However, as the limits of investigation werepushed out into the infrared beyond about 2.7 microns, the glass prismbecame gradually, sometimes suddenly, opaque in these regions, andinvestigators found it necessary to go to such materials as quartz, rocksat, or potassium bromide to secure the transparency coupled withdispersive power necessary to make an effective prism. Each of thesematerials shows high dispersion or a rapid change of refractive indexwith wave length in some particular region with a gradual decrease ofdispersion in other regions. Generally, the highest dispersion isassociated with approach to a region of high absorption, hence theregion in which the prism separates wave lengths best is also the regionin which it begins to refuse to transmit. Trouble then results; eitherthe user must be content with low dispersion but good transparency ofthe prism, or he finds that the thicker part of his prism begins tobecome opaque as the wave length changes, passing the beam only throughthe thin part or vertex of the prism; eventually the whole prism failsto transmit. This same state of affairs manifests itself as measurementsare pushed farther into the shorter ultraviolet wave lengths, glasscutting oti at about 300 millimicrons and quartz in the vicinity of 160to 200 millimicrons. Further down, to about millimicrons, lithiumfluoride may be used. The relationship between dispersion and proximityto the absorption limits holds in this region also.

The increasing availability of good diffraction gratings as primarydispersing means with their superior and more constant dispersionrelieved the situation for users of single monochromators but theproblem remains for spectrophotometers. Gratings produce a plurality ofoverlapping spectra which interfere with one another inspectrophotometric measurements. The ideal way of removing all exceptthe desired spectral order is a second monochromator, but a gratingcannot be used as the secondary dispersion means because it will notpass just one order. So the problem of limited prism range and of widelyvarying dispersion and distortion of the optical beam near absorptionlimits is still a factor in the design of the second monochromator.Also, especially in the infrared, the desirability of using very largebeam cross sections for increasing response increases the size ofcomponents and requires that the prism be a large costly item. The largesize of the prism, of course, aggravates the problem of absorption atspectral extremes and at any absorption bands the prism material maypossess in mid range.

An important object of the invention is to provide a small size, lightweight, readily and simply mounted, and low cost optical prism unit. Aparticular object is to provide such a unit which is adapted for use ina spectrophotometer, in monochromators in general, and in spectrographs.

Another object is to provide a prism unit which is characterized byincreased optical efficiency, and particularly by high dispersioncoupled with low absorption.

Another object is to provide a unit which furnishes increasedhomogeneity of optical beam cross section.

A further object is to provide a unit which reduces the amount of straylight in the system.

Other objects include the provision of a unit adapted for increasing theavailable dispersion, widening the operating range, synthesizing newdispersion curves previously unobtainable, and reducing the number ofoptical components in the system.

Further objects include the provision of new and improved methods ofmanufacturing the optical prism unit.

These and other objects, advantages and functions of the invention willbe apparent on reference to the specification and to the attacheddrawings illustrating preferred embodiments of the invention, in whichlike parts are identified by like reference symbols in each of the viewsand in which:

FIGURE 1 is an end elevational view of a prior art prism and a prismunit according to the invention outlined thereon;

FIGURE 2 is a cross-sectional view of a monolithic form of the new prismunit;

FIGURE 3 is a cross sectional view of an embodiment of the prism unitincluding a matrix adhered to individual prisms thereof;

FIGURE 4 is a perspective view of the embodiment of FIGURE 3;

FIGURE 5 is a cross-sectional view like FIGURE 3, illustrating the useof a backing plate or frame on the unit;

FIGURE 6 is a modification of the embodiment of FIGURE 5 in which thebases of the individual prisms form light traps;

FIGURE 7 is a cross sectional view of another embodiment of the prismunit which functions as a light trap;

FIGURE B is a cross-sectional view of an embodiment of the prism unitadapted to focus an optical beam;

FIGURE 9 is a perspective view of an embodiment similar to FIGURE 8illustrating axial curvature of the individual prisms;

FIGURE l is a schematic elevational view illustrating apparatus for anda method of manufacturing the embodiment of FIGURE 2;

FIGURE ll is a schematic plane view illustrating a method ofmanufacturing the embodiment of FIGURE 7:

FIGURES l2 and 13 are side elevational views, and FIGURE 14 is across-sectional view illustrating apparatus and a method formanufacturing a prism unit such as illustrated in FIGURE FIGURES 15 and16 are fragmentary side elevational views similar to FIGURES 12 and 13illustrating a method of manufacturing a prism unit having curved prismbases;

FIGURE 17 is a schematic view showing a prism unit such as illustratedin FIGURE 3, employed in a simple Littrow monochromator;

FIGURE 18 is a schematic view showing two prism units employed in adouble dispersion Bunsen monochromator; [and] FIGURE 19 is a viewsimilar to FIGURE 5 showing an opening through a matrix and backingplate; and

FIGURE 20 illustrates multi-prism translation laterally as themulti-prism is rotated to change wave length.

As the most elementary basic form of the invention, the bulky prior artprism 10 is replaced by a light, thin multi-element prism 12 outlinedthereon, as shown in FIGURE l. In this figure, the sides 14, 16 and 18represent respectively the transparent hypotenuse, the ground base andthe reflecting back face of a conventional Littrow prism. Thecorresponding lettered numbers 14a, 16a and 18a, represent thecorresponding components of a plurality of prism elements which,properly aligned, will perform all the functions of the prior art prismin a superior manner. When these elements are made up, either as amonolithic unit or as an assembly held in a suitable matrix as shown inseveral forms in the successive illustrations, the basic, simplest formof the invention is provided which, for the sake of simplicity, will becalled the multiprism This basic multiprism may be made with sides 14acoplanar as shown, with other sides such as 18a coplanar, or, as will befurther discussed hereinafter, it may be desirable to use otherorientations for their peculiar advantages. In the course of thisdisclosure, a particular arrangement may be referred to for purposes ofillustration, but it is to be understood that the invention is notlimited thereto.

Referring to FIGURE 2, which shows the multiprism as a monolithic form19, surface 14b is the entrance face as in FIGURE 1, faces 16b are thebases of the successive prism components, and surfaces 18b are the exitor reflecting faces. The light path is illustrated by the arrows 21.Both of the surfaces 14b and 18h are polished accurately hat to a degreerequired for the specific use and in accordance with the usual practicesin the art. The interval between corresponding parts of adjacent prismelements, or prismlets, may be of the order of 0.1 inch, although largeror smaller spacings may be used as required. The thickness of this rorinis somewhat greater than subsequent forms to be discussed on account ofthe necessity of providing rigid attachment between prismlets; this isone disadvantage of this form. Another is the diiculty of polishing. Forthese reasons, the preferred embodiments lie in the followingconstructions.

In FIGURES 3 and 4, the prismlets 20 are separate pieces supported by amatrix 22. This matrix may be of a variety of materials, preferablychosen to have a temperature coefficient of expansion approximating thatof the prismlets, and may be formed by the same process as used to roughout monolithic form 19, described subsequently. Alternatively, thematrix may be a metal investment casting or a forged or molded unit. Inany of these cases, the prismlets are attached to the matrix bycementing with a suitable cement such as an epoxy resin. The entirematrix may be an epoxy casting, silica filled and cast in place, and mayalso have sockets or mounting studs cast into it.

Variations of the above embodiment are shown in FIG- URES 5 and 6. Inthese the rigid back is a flat plate or frame 24 attached to theprismlets by casting a rigid adhesive material 26 such as epoxy resinonto the prismlet assembly and attaching the plate, preferably beforecure takes place. The matrix 26 embraces the faces 16c, d, and 18e, d,of the individual elongated prisms. FIG- URE 6 illustrates also that theangle and shape of the bases 16d of the prismlets may be altered fromthose in the preceding views to form a light trap for the light thatwould otherwise scatter from the base.

This principle of the light trap is further exemplified in theembodiment shown in FIGURE 7. This multiprism requires no matrix and ismade by a different process from either of the two embodiments above.The prismlets in this form are not triangular but behave optically as ifthey were, the difference being that the former base indicated in brokenline at 16e has been transformed into a roughly triangular pocket whichforms an effective light trap. This is illustrated by the arrow 27. Theprismlets are provided with a reliective coating 28 on their back faces18e, and they are cemented to each other as indicated at 30 along theback faces. A notch 32 filled with this cement strengthens the structureand improves the light trapping.

The individual prisms in the prism units or multiprisms illustrated inthe several views function together as a single prism. The front lightreceiving faces 14a, 14h, etc., are oriented for exposure to a lightsource. The front entrance faces and the back cooperating faces 18a, 18hetc. are oriented for functioning as a single prism. In the embodimentsof FIGURES 2-7, they provide parallel refraction in the same directionof light of the same wave length. In FIGURES 2-6 and 19, the backcooperating faces may reflect or transmit light. In the latter case byproviding an opening 25 in the matrix 22 or 26, and in the plate 24 asshown in FIGURE 19. Reflection may be effected in FIGURES 2-7 by coatingthe back faces or providing a reflective surface behind them. The prismspreferably are aligned in vertex-to-base relationship, and with thefront, light entrance faces lying in a common plane, in the mannerillustrated.

The basic multiprism in any of the forms of FIGURES 2-7 possesses anumber of advantages over prior art prisms. The whole section ofmaterial beyond the outline in FIGURE 1 has been eliminated. Since someprism materials are expensive, this represents a considerable saving incost and, in the case of some materials which are hard to secure inlarge, unawed pieces, this may make a formerly prohibitively pricedprism possible.

A further advantage of the multiprism is its low weight compared to theconventional prism. This minimizes problems in instrumental design ofmounting, pivoting, guiding, and protecting from mechanical damage.Furthermore, the overall shape and construction of the multiprism ismore amenable to simple mounting methods.

Another important advantage of the multiprism is its optical thinness Itwill be apparent on inspection that the total distance through which theoptical beam travels in the material of the multiprism is less at anyplace across the whole width of the multiprism than it is at any placeacross the face of the conventional prism except near the vertex. Thismeans that the user of the prism can approach much closer to theabsorption limits of the material without excessive loss of light in themultiprism than he can in the prior art prism. It further means thatwhen the absorption limit is approached, the beam is attenuated evenlyover its whole width rather than being progressively darkened from theedge adjacent to the prism vertex. Since in many spectrophotometers anonuniform darkening of the beam introduces serious photometric errors,a serious source of instrumental error is thus eliminated by themultiprism. Furthermore, some very desirable optical materials such assynthetic quartz have impurity bands which prevent them from being usedin certain wave length regions in the thickness required by conventionalprisms; with the multiprism, their use becomes entirely practical. Inthe case of materials prone to internal scattering, the optical thinnessof the multiprism is a great aid to the reduction of scattering electssuch as stray light error. This reduction of stray light is furtheraided by the elimination of scattering from the base in theconstructions shown in FIGURES 6 and 7, where the base has been movedout of the boundaries of the conventional prism to where it cannot be asource of scattering and where, in fact, the pocket so formed becomes alight trap to catch and suppress scattered light in a way not effectivein the conventional prism.

The advantages of the new multiprism are illustrated i in themonochromator embodiments of FIGURES 17 and 18. In the simple Littrowmonochromator of FIGURE 17, the entrance slit is indicated at S1 and theexit slit at S2. Two curved mirrors M1 and M2 are interposed in thelight path. The multiprism MP is rotated as indicated at MP to vary thewave length. In the double dispersion Bunsen monochromator of FIGURE 18,Sx and S2 are the entrance and exit slits, respectively. Two lenses L1and L2 are interposed in the light path. The two multiprisms MP1 and MP2require less room than one conventional prism. The multiprism is alsoemployed advantageously in spectrophotometers and in spectrographs.

A series of modifications of the basic multiprism may be made, whichgive it additional advantages not possible in any prior art prism. Thelrst of these is the use of a plurality of materials in the severalprism elements. As a simple example of this, half of the elements can bemade of synthetic quartz and half of another fused quartz, the

former having very high transmission in the range from 160 millirnicronsup through the visible but having a pronounced absorption band at 2.7microns, and the latter having little transmission under 200millimicrons but haw ing no absorption hand at 2.7 microns. Both havethe same dispersion curve to a sufficient degree of accuracy lli for usein a secondary dispersing means. In the simplest form, alternatingprismlets of the two materials are used. The resulting multiprismtransmits well over the total range from 160 to weil above 4000 mma. Nosingle prior art fused quartz prism has this uninterrupted wave lengthrange. This arrangement does attenuate alternate strips of the beam inthe low transmission regions of one or the other material and thus loselight, although the beam remains homogeneous to a degree impossible ifit were attempted to put two conventional prisms side by side, andfurthermore there is no hole in the center of the beam such as would beproduced by the thick base ot' one of the conventional prisms. There is,however, a more sophisticated arrangement to which the multiprism iswell adapted which does not result in a loss of light. All thesynthetic, high UV quartz can be placed at one end and all thc high IRquartz at the other end of the multiprism, and the multiprism is thentranslated laterally as it is rotated to change wave length. In the UV,the beam strikes only on the synthetic `iuartz, while in progressing tothe 1R, the beam moves across the multiprism to the region composed onlyof high IR quartz. This cannot be done with conventional prisms withoutcausing an objectionable dark strip to move across the beam in midrange. So by using a multiprism of two materials and translating as wellas rotating it, an eicient range of operation can be achieved which isimpossible with prior art prisms.

This principle can be further developed. The multi prism can be made oftwo or more materials of different dispersion curves, so as to produce anew synthetic dispersion curve not found in any known material. This isdone by placing some such material as quartz at the UV end of the prismso as to use its high dispersion range. As 300 mma is approached themultiprism is gradually changed into glass with a different prism angleto match the deviation of the quartz. Above 300 mma, the glass takesover but has improved dispersion over quartz, the beam leaving thequartz as the multiprism translates and passing onto the glass section.This process can be extended from one material to another as far asdesired. As a secondary dispersion means, the doubling of the beambetween regions where the deviation curves draw apart is not serious.The ability to maintain high dispersion over a greatly extended range isunknown to prior art prisms and is a novel advantage of the multiprism.

An additional variant on the above form of multi prism involves the use0f selective multilayer rellecting coatings of the broad band type onthe rellecting surfaces of the prism elements instead of simplealuminizcd surfaces. These coatings are selected to reect in theparticular wave length region for each prismlet that the particularprismlet is to transmit. As the beam moves from, said quartz to glasswith increasing wave length, the coating on the back of the quartzsections cuts oli .ind ceases reflecting at the increased wave lengths,while at the same time, the coatings on the glass cut in, thuseliminating the doubling of the beam due to the drawing apart of thedeviation angles for the quartz and for the glass. This feature adaptsthe multiprism for use as a primary dispersion means.

All the foregoing are variants on the basic multiprism with planeentrance and reection faces. It will be apparent that the scope of themultiprism principle is very wide and limited only by the ingenuity andrequirements of the designer of the particular multiprism. It will alsobe apparent to one skilled in the art that the above description doesnot exhaust the possible variants on this principle even for the basicmultiprism. For brevity. the description will now be directed tovariants of thc` multiprism which include its secondary function ofchanging the convergence or divergence of the beam. This functioninvolves the use of surfaces on the prism elements which have power,i.e., are curved rather than plane.

gaat? The classic conventional or prior art prism which has power is theFery prism, having curved front and back faces. Its purpose is todispense with a collimator mirror having a spherically curved surface,the function of forming an image of the entrance slit at the plane ofthe exit slit normally performed by such a curved collimator mirror nowbeing performed by the curved surfaces of the prism. As used by Fery andlater in a commercial spectrophometer, its only advantage was somereduction in chromatic aberration from that caused by a sphericalcollimator with an olf-axis prism. Properly the faces of this prismshould be based on the logarithmic spiral but for practical purposes thesurfaces are spherical. Short radii, large aperture ratios and shortfocal lengths are not very practical with this prism and a seriousdefect of the prism is its large astigmatism which causes points on theentrance slit to be imaged as lines of some length at the exit slit.Because of these disadvantages, it has still proved expedient when usingthis prism to fold the optical system to keep the instrument from beingtoo long,

so that the cost of the collimator mirror has not been eliminated; ithas simply been changed into that of a fiat mirror, which is essentiallythe same.

A multiprism which also performs the functions of the Fery prism isshown in FIGURE S. The corresponding curved faces are numbered asbefore, 14f and 18f. The bases 16f may, as in the basic multiprism, beof any shape. While FIGURE 8 shows surfaces 18f as being segments of theback face of the Fery prism, it will be apparent to one skilled in theart that a more suitable shape may be used if required. It will befurther apparent that the multiprism may be bent in either direction,e.g., as illustrated for the face surface by the broken line 34, thusallowing the designer a greater degree of freedom than that afforded bythe prior art prism to reduce the aberrations previously mentioned ascharacteristic of the Fery prism. It also will be apparent that theprism elements need not be of identical width, nor need they even belinear axially if an axially curved element is better. This latter pointis illustrated in FIGURE 9. A multiprism 36 includes curved entrancesurfaces 14g `and reflecting surfaces 18g, and axial curvature of theelongated elements 38. The curvature of the surfaces may be spheric oraspheric.

The embodiments of FIGURES 8 and 9 have their front faces 14f, 14g andtheir back faces 18f, 18g oriented for refraction in the same directionto focus light of the same wave length. The front faces of theindividual prisms lie in a common curved surface on each prism unit.

Since the curved multiprism is more flexible from a design standpointthan the Fery prism, it may be made to a shorter focal length and maydispense with both collimator and plano folding mirror. This eliminatesfrom the system two refiections with their losses and liability toscattering errors. It reduces the cost of the system. It furthersimplifies realization of the wider range of operation and the otheradvantages enumerated above for the basic multiprism, alsocharacteristic of the curved surface prism.

The optical prism units may be manufactured in several ways. Threemethods of construction are illustrated, having an increasing degree offiexibility.

The rst method results in a one piece or monolithic basic multiprism.Referring to FIGURE 10, a fiat blank 46 of the material to be used,e.g., quartz, is ground and polished optically fiat on one surface, thenblocked onto a supporting base 40. The base may be a fiat metal plate onwhich the blank is adhered in the conventional manner using pitch orother suitable adhesive. The blocked piece is then. placed in a machineof the type known commercially as the Cavitron, which is equipped withan ultrasonic power unit which can vibrate a tool 42 in the direction 44normal to the piece at a frequency of several tens of kilocycles persecond over an amplitude of several hundredths of an inch or less. Thetool has previously been shaped by conventional grinding methods to aform which is thc reverse of that desired on the multiprism. A typicalmaferial for the tool is 440C stainless steel hardened to about 45Rockwell C. By feeding a suitable abrasive such as boron nitride betweenthe tool 42 and the work 46, the work will rapidly take the shape of thetool being fed into its surface, the time required to complete theforming of the work being one or two minutes. Polishing requires aloaded or hard pitch tool of the same shape as the work. This may bepressed to shape on the work or, better, on an accurate form havingseveral more steps than the number of prismlets of the work. This toolis charged with Barnesite or other suitable polishing material andworked against the work in narrow' random ellipses whose minor axis isas shown at 48 and whose major axis is parallel to the grooves in thework. At intervals the polishing tool is moved laterally the distance ofone groove interval. The amplitude of motion of the tool must be smallto avoid rounding the edges of the faces. Ultrasonic drive of this toolgreatly reduces the polishing time. It is also possible to provide apseudopolish with a metal lap of the proper shape charged with diamondpolishing compound. After polishing, the surfaces 18h may be given theproper reflective coat as a last step.

The difficulty of maintaining the flatness of the faces 18b during thepolishing operation is a limitation of this method of manufacture, whichis overcome in the following methods producing the nonmonolithicstructures.

To arrive at the form of multiprism shown in FIGURE 7, a number ofpieces or sheets of the prism material are ground and polished plano tothe required accuracy on one face, and are ground plane parallel but notnecessarily polished on the other face. Referring to FIG- URE ll, theseelements are indicated at 50, the polished face of each being on theunder side as shown at 18e. Each piece is long enough to produce severalmultiprisms. The degree of plane parallelism between the upper and lowerface of each piece depends on the accuracy to which the vertex angles ofthe prismlets must be held, but this can be more easily controlled thancan the vertex angle of the conventional prism. Each piece is aluminizedor otherwise refiectively coated on the polished side 18e tosubsequently give the reecting face of the multiprism. A plurality ofgrooves 32 of optional shape are then cut in each piece to form anchorsfor the cement holding the assembled multiprism together and also toimprove the effectiveness of the light trapping. These grooves may beomitted without invalidating the process, or additional grooves may beadded either in the bottom or top face of the piece, the onlyrestriction being that these grooves do not enter the area of face 18e0n the bottom side of the piece nor enter the volume of the prismletdefined by faces 14e, 18e and the dotted line 16e at the lower rightcorner of FIG. ll. After grooving, the pieces are laid up into a skewedpile or stack as shown, the angle of skew being approximately the vertexangle of a prismlet. Between each layer is a thin film of a cement suchas a low viscosity, room temperature curing epoxy. The stack is allowedto cure without heating or applying any pressure which would causestrains to be set up in the elements. After cure the stack is sawed intotransverse slabs, along parallel planes 52, using a diamond saw, theangle of the planes being held to as near that of the vertex angle aspractical. The resulting slabs are reblocked to give the exact vertexangle between faces 18e and the top faces 14e when the face surface isground plano and polished as the final operation.

This second method depends on the plane parallelism of the initialpieces. The process is somewhat wasteful of material and best adapted tothe particular form of multiprism shown in FIGURE 7. The followingmethod of manufacture 0f the multiprism is far more flexible and lessexpensive.

The basic principle which must be followed to secure :in accuratefinished multiprism is that the individual ele ments must be undercomplete control as to position from the start of the fabricationprocess to the finished unit. As these pieces are small and easilydistorted they must at all times be held by a rigid. larger piece thatcan be guided, and held in position. FIGURES 12-16 illustrate the methodused to effect this control. Referring to FIG- URE l2, a stack of metalsupporting plates 54 is employed, which includes plates that may beabout 3 inches long by 2 inches wide by 0.1 inch thick. These plates ordops" as referred to in the gem cutting industry, are clamped betweentwo heavier metal end members 56 and S by suitable means such as athrough bolt 60 passing through the stack. The plates and end membersare all finished on all faces to be accurately rectangular, and theadjoining surfaces are accurately ground plane parallel. Theillustrative equipment is designed for making a basic multiprism; for amultiprism with curved surfaces, the tooling is appropriately modified.

To the top of each supporting plate 54 is attached by blocking pitch orother suitable material a rod 60 of the material from which themultiprism is to be made. In the most elementary form, this rod is thesame size as the top of the plate and of somewhat greater thickness thanthe final prismlets base width. It may be preground to size or attachedfirst to the plate and then ground to correspond to the-thickness of theplate. The assembly is wrung down on a surface plate so that the bottomsurface 62 is flat before clamping. Suitable protecting bars 64 areadded around and surface 66 is polished plano to the required degree ofaccuracy. The proper reflecting coat is then applied to sunface 66 ifdesired. This surface will become the prism back faces 18e.

The stack is next unclamped and reassembled with skewed end pieces 68and 70, with the buttom end of every supporting plate squarely incontact along the edge thereof with a supporting surface 72 which isaccurately plane parallel with the base surface 74 on the end piece 68.The angle of skew must accurately be the vertex angle to be formed onthe prismlets. The accuracy of alignment of the prism faces 18C can beeasily checked by optical methods before the next step and any necessarycorrections made, A suitable dam 76, which may be rubber, Teflon, orother nonsticking material, is placed around the area of the skewed,coated prism blanks. The hardenable fluid matrix material 26, e.g.,epoxy resin material, is cast into this area and a backing plate 24adhered, if used. A transparent multiprism can alternatively be made foruse as a Bunsen prism or with a separate Littrow mirror by omitting areflecting coating and blocking olf the central part of the area toconfine the matrix to a iframe around the central clear aperture 25shown in FIGURE 19. In either case, the matrix is cured at roomtemperature to avoid strains in the assembly.

The back 78 of the matrix backing plate can then be ground parallel tobase plane 74 which is plane parallel to the supporting surface 72, tosimplify the establishment of the vertex angle in the final step. Thisoperation can be omitted and shimming used in the nal step with anoptical test for angle. The clam-ps are next removed and the supportingplates 54 separated from the cemented assembly by chilling or othersuitable means, leaving the assembly shown in FIGURE 14. As the laststep, this is blocked against the plate back 24 and ground and polishedto a plane 80, parallel to the back 78, to complete the multiprism.

The above method is readily adaptable, as will be apparent to oneskilled in the art, to multiprisms with curved bases. For example, byinserting a plane parallel shim 82, as is shown in FIGURE 15, it ispossible to start with round rods 84 which are much easier to secure insome materials such as fused quartz. In this case the rods arecenterless ground to equal size before attaching to the supportingplates 54, tangent to one face 84 on each plate. After the rods areground flat to the diameter and reflectively coated, when desired, toproduce [lat back laces 18h. the plates are skewed as in FIGURE i6 withthe shims removed. The ground rods are embedded in the matrix and nishedas described for the embodiment of FIG- URES 13 and 14, grinding therods to a plane 86 to provide front faces 14h on the resulting prisms.The final prisms have curved bases 16h which form excellent light traps.

It will be apparent that the optical prism unit and the methods ofmanufacture may be changed and modified from the preferred embodimentsdescribed and illustrated, within the spirit and scope of the invention.It is intended that such changes and modifications be included withinthe scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An optical prism unit for dispersing rays of light characterized bylight weight, low light absorption, and high resolution whichcorn-prises a plurality of thin individual elongated prisms, eacl` prismhaving sides joining at one apex thereof ground and polished opticallyhat to serve as entrance and cooperating faces respectively for lightrays to be dispersed, and a base face opposite said apex, said prismsbeing embedded in a matrix in side by side alignment with said lightentrance faces and said apexes of each disposed in a single opticallyfiat plane which forms the light receiving face of the prism unit, saidmatrix embracing said cooperating and base faces to secure permanentlythe prisms together and being so disposed within said optical unit thatthe light rays do not enter the matrix material.

2. The optical prism unit of claim 1 in which said cooperating face iscoated with a light reflective material so that the light rays arereflected without entering said matrix.

3. 'The optical prism unit of claim 1 in which said matrix embraces saidcooperating and base faces near the extremities of said elongated prismsto leave an area free of matrix material so that light rays may passdirectly from the prism unit through said cooperating faces in said areawithout entering the matrix material.

4. The optical prism unit of claim 1 in which said individual prisms aremade from different materials having distinct light dispersingproperties to provide a prism unit which functions effectively over abroad range of wave lengths.

5. An optical prism unit for dispersing rays of light characterized bylight weight, low light absorption and high resolution which comprises aplurality of individual elongated dispersing prisms arranged side byside in a matrix in a linearly extended array, each prism having twocurved sides joining at one apex thereof ground and polished to apredetermined curvature to serve as entrance and cooperating facesrespectively for light rays to be dispersed, and a base face oppositesaid apex, the angle subtended by the polished faces of each prism beingof varying magnitude so that said light rays having a common Wave lengthare refracted to a common focus peculiar to that particular wave length,said entrance faces of each prism being disposed in a common curvedsurface, said matrix embracing said cooperating and base faces to securepermanently the prisms together and being so disposed within saidoptical unit that the light rays do no enter the matrix material.

6. The optical prism unit of claim l in which said plurality of prismshas progressively variable wave lengt/1 dependent characteristics toprovide maximum energy output over a range 0f wave lengths from infraredto ultra violet by translating and rotating said unit relative to saidrays of light to control the intensity and dispersion of said rays oflight.

7. An optical prism unit for dispersing rays of light comprising, incombination,

a plurality of groups of dispersive optical elements of prismatic crosssection, each of said groups having l l l 2 individually differentiatedwavelength dependent of record in the patented le of this patent or theoriginali characteristics from infrared t ultraviolet, said patent.

groups being mounted on a carrier means in the order UNITED STATESPATENTS of progressively variable wavelength dependent Chf- 312,29()2/1385 pgnycuick g8 50 acteristics from infrarfd t0 LllIrVO/C l0 PI'OVdE5 578,620 3/1397 Barker 88 57 maximum efficiency over a range ofwavelengths from 596333 1 1898 jabs 38 611 infrared t0 ultraviolet bytranslating and rotating Said 982,772 1/1911 Hadsworth 8;; 60

plurality of elements of prismatic cross section rela- 1,434,167 /1922Thomative to rays of light t0 Contrl the intensity and dS- 1,806,8645/1931 pallemaerts 88 1 persian Ofwid "WS 0f light 1g 1,872,501 8/1932Rehlanaer ss-i said elements of prismatic cross section for dispersion2,394,645 2/1946 Turner et a1- 51 216 of rays of light beingcharacterized by light weight 2,466,455 4/1949 Luboshez 88 1 and lowlight absorption, each element Comprising at! 2,499453 3/1950 Bonnet 881 X individual elongated dispersing prism side by side 807922 10/1957Newcomer et a1 51 281 with adjacent elements in a linearly extendedarray 2,855,819 10/l958 Luboshez 8;; 1

in a matrix in said carrier means, each prism having 3,062,089 11/1962Martin 35(3 162 two polished sides joining at one apex thereof to serveas entrance and cooperating faces respectively FOREIGN PATENTS for lightrays to be dispersed, and a base face 0pposite said apex, the anglesubtended by the polished 1 grendm' faces of each prism being of varyingmagnitude so 1089'184 5/1960 Germarry that said light rays having acommon wave length are 376'982 .H1932 Great Bri'rain redirected in acommon direction peculiar to that par- 458509 12/1936 Great Britarrticular wave length, said entrance face of each ele- 90,535 8/1962 GreatBritain' ment being disposed in a common plane surface, said matrixembracing said elements to secure the ele- DONALD L WIBERTPrr-maryBrummen ments permanently together and being so disposed within saidoptical unit that the light rays do not W- L- SIKESSSISU'I?Examinerenter the matrix material.

U.S. Cl. X.R.

RefelellCeS Cited 350 286 320; 356 98 The following references, cited bythe Examiner, are

